It is known in such an assembly, there is an operation frequency of the synchronous machine below which the machine bridge can no longer operate without forced turn-off or extinction of the thyristors of the machine bridge. In general the installation is then made to function according to the mode sometimes called the "sequential" mode which is described hereinbelow. In the sequential mode, when it is necessary to change from one configuration of the machine bridge to another, the two thyristors which were conductive are extinguished by inverting the polarity of the mains bridge before the two thyristors which correspond to the new configuration of the machine bridge are turned on. Now, each time the machine bridge changes configuration, one of the two thyristors which was in the conductive state before the change is also in the conductive state after the change; its extinction is therefore useless and even detrimental, since during the period when both thyristors are off, no current flows in the machine and its torque is then zero.